1. Field of the Invention
The invention relates generally to an electrophysiological (xe2x80x9cEPxe2x80x9d) system for providing radio frequency (xe2x80x9cRFxe2x80x9d) energy to biological tissue and, more particularly, to an ablation system having multiple selectable current path means for directing the flow of current through tissue in energy and time efficient manners.
2. Description of Related Art
The heart beat in a healthy human is controlled by the sinoatrial node (xe2x80x9cS-A nodexe2x80x9d) located in the wall of the right atrium. The S-A node generates electrical signal potentials that are transmitted through pathways of conductive heart tissue in the atrium to the atrioventricular node (xe2x80x9cA-V nodexe2x80x9d) which in turn transmits the electrical signals throughout the ventricle by means of the His and Purkinje conductive tissues. Improper growth of, or damage to, the conductive tissue in the heart can interfere with the passage of regular electrical signals from the S-A and A-V nodes. Electrical signal irregularities resulting from such interference can disturb the normal rhythm of the heart and cause an abnormal rhythmic condition referred to as xe2x80x9ccardiac arrhythmia.xe2x80x9d
While there are different treatments for cardiac arrhythmia, including the application of anti-arrhythmia drugs, in many cases ablation of the damaged tissue can restore the correct operation of the heart. Such ablation can be performed by percutaneous ablation, a procedure in which a catheter is percutaneously introduced into the patient and directed through an artery to the atrium or ventricle of the heart to perform single or multiple diagnostic, therapeutic, and/or surgical procedures. In such case, an ablation procedure is used to destroy the tissue causing the arrhythmia in an attempt to remove the electrical signal irregularities or create a conductive tissue block to restore normal heart beat or at least an improved heart beat. Successful ablation of the conductive tissue at the arrhythmia initiation site usually terminates the arrhythmia or at least moderates the heart rhythm to acceptable levels. A widely accepted treatment for arrhythmia involves the application of RF energy to the conductive tissue.
In the case of atrial fibrillation (xe2x80x9cAFxe2x80x9d), a procedure published by Cox et al. and known as the xe2x80x9cMaze procedurexe2x80x9d involves continuous atrial incisions to prevent atrial reentry and to allow sinus impulses to activate the entire myocardium. While this procedure has been found to be successful, it involves an intensely invasive approach. It is more desirable to accomplish the same result as the Maze procedure by use of a less invasive approach, such as through the use of an appropriate EP catheter system.
There are two general methods of applying RF energy to cardiac tissue, unipolar and bipolar. In the unipolar method a large surface area electrode; e.g., a backplate, is typically placed on the back of the patient to serve as a return. The backplate completes an electrical circuit with one or more electrodes that are introduced into the heart, usually via a catheter, and placed in intimate contact with the aberrant conductive tissue. In the bipolar method, electrodes introduced into the heart have different potentials and complete an electrical circuit between themselves. In the bipolar method, the flux traveling between the two electrodes of the catheter enters the tissue to cause ablation.
During ablation, electrodes carried by an EP catheter are placed in intimate contact with the target endocardial tissue. RF energy is applied to the electrodes to raise the temperature of the target tissue to a non-viable state. In general, the temperature boundary between viable and non-viable tissue is approximately 48xc2x0 Centigrade. Tissue heated to a temperature above 48xc2x0 C. becomes non-viable and defines the ablation volume. The objective is to elevate the tissue temperature, which is generally at 37xc2x0 C., fairly uniformly to an ablation temperature above 48xc2x0 C., while keeping both the temperature at the tissue surface and the temperature of the electrode below 100xc2x0 C. When the blood temperature reaches approximately 100xc2x0 C., coagulum can occur. Blood coagulation is a major limitation or complication associated with RF ablation therapy. Coagulation can lead to thromboembolism and also form an insulating layer around the electrode, thereby hindering further energy delivery required for ablation therapy. Thus, heating of blood is a major concern for ablation safety and efficacy.
A basic configuration of an RF ablation system, as shown in FIG. 1, includes an ablation catheter and a backplate. The ablation catheter includes a distal tip which is fitted with a tip electrode for applying RF energy. The tip electrode is the source of an electrical signal that causes heating of the contacting and neighboring tissue. The tip electrode act as one electrical pole. The other electrical pole is provided by the backplate which is in contact with a patient""s external body part. The backplate acts as a current path means in that it establishes a current path with respect to the tip electrode. During operation, a RF signal, typically in the 500 kHz region, is applied to the tip electrode. The current path for the RF signal established between the tip electrode and the backplate produces a localized RF heating effect in the tissue which in turn produces a lesion. In order to obtain a deep, localized lesion at the contacting tissue, i.e., target tissue, using a given amount of RF energy, the backplate and tip electrode should be optimally positioned relative each other, not as shown in FIG. 1, such that the target tissue is located between the tip electrode and the backplate. However, the backplate may not always be located in the optimal position. Specifically, if the backplate is positioned such that the target tissue is not between the ablating tip electrode and the backplate, as shown in FIG. 1, the current that enters the tissue conducts away from the tissue interior toward the backplate, causing more energy to be dissipated into the blood. The net result of the redistribution of current is a measurable difference in lesion depth and size. In order to obtain a lesion of quality similar to that obtainable when the backplate is in the optimal position, the amount of current flowing through the tissue must be increased. However, this presents a risk that the blood may be heated to the point where coagulum forms (100xc2x0 C.) before achieving a desired lesion volume. Alternatively, the backplate may be physically repositioned on the patient in order to place it in the optimal position. This is undesirable for the patient because it increases the overall time of the procedure.
Hence, those skilled in the art have recognized a need for an ablation system having multiple and selectable current path means for directing a current through target tissue to thereby create a quality lesion in a safe and efficient manner. The invention fulfills these needs and others.
Briefly, and in general terms, the invention is directed to a system and method for selectively providing RF energy to biological tissue and, more particularly, to an ablation system having multiple selectable current path means for directing the flow of current through tissue in energy and time efficient manners.
In one aspect, the invention relates to a system for delivering energy to any one of a plurality of tissue regions at a biological site within a biological body. The system includes at least one primary electrode that is adapted to be positioned adjacent any one of the plurality of tissue regions. Also included in the system is a plurality of secondary electrodes adapted to be positioned about the biological site such that any one of the plurality of tissue regions is interposed between at least one of the secondary electrodes and the at least one primary electrode. The system also includes a power generator that is adapted to provide power to the at least one primary electrode. Further included is a switching device that is adapted to selectively couple one of the secondary electrodes to an electrical device. The electrical device is adapted to maintain the selected secondary electrode at a voltage level that is different from the primary electrode such that current flows from the primary electrode through the tissue region to the selected secondary electrode.
In a detailed aspect, the at least one primary electrode within the system is disposed at a distal segment of a first catheter. The first catheter is adapted to be positioned within the biological body. In a further aspect, at least one of the secondary electrodes includes a backplate that is adapted to be positioned adjacent the exterior of the biological body. In another aspect, at least one of the secondary electrodes is disposed at a distal segment of a second catheter that is also adapted to be positioned within the biological body. In an additional aspect, the primary electrode and at least one of the secondary electrodes are disposed at a distal segment of a single catheter that is adapted to be positioned within the biological body.
In an additional facet, the system includes a controller that is adapted to select one of the secondary electrodes for coupling with the electrical device and to command the switching device to couple the selected secondary electrode to the electrical device. In one detailed facet, the controller is adapted to measure the impedance between each of the secondary electrodes and the primary electrode, compare the impedances and based on the comparison, select the secondary electrode for coupling to the electrical device. In a further detailed facet, in order to measure the impedance between each of the secondary electrodes and the primary electrode, the controller is adapted to apply an electric pulse at the primary electrode, sense the electric pulse at a first of the secondary electrodes, calculate the difference between the voltage of the pulse signal and the voltage of the sensed signal to determine the impedance, and repeat the process for each of the remaining secondary electrodes. In another detailed aspect, the controller is adapted to measure the conduction time of a pulse between each of the secondary electrodes and the primary electrode, compare the conduction times and based on the comparison, select the secondary electrode for coupling to the electrical device. In a further detailed aspect, the secondary electrode with the shortest conduction time is selected.
In a second aspect, the invention relates to a system for providing a plurality of selectable current paths through a plurality of tissue regions within a biological site within a biological body. The system includes at least one primary electrode that is adapted to be positioned adjacent any one of the plurality of tissue regions and to serve as a first electrical pole. The system also includes a plurality of secondary electrodes adapted to be positioned about the biological site such that any one of the plurality of tissue regions is interposed between at least one of the secondary electrodes and the primary electrode. Each of the secondary electrodes is adapted to be selected to serve as a second electrical pole having a voltage different than the first electrical pole such that current flows through the tissue region between the first and second electrical poles. Further included in the system is a selection system adapted to select one of the secondary electrodes as the second electrical pole.
In a detailed aspect, the selection system includes a switching device that is adapted to selectively couple one of the secondary electrodes to an electrical device. The electrical device is adapted to maintain the selected secondary electrode at a voltage level different than the primary electrode. In a further aspect, the selection system includes a controller that is adapted to select one of the secondary electrodes for coupling with the electrical device and to command the switching device to couple the selected secondary electrode to the electrical device. In a detailed aspect, the controller is adapted to measure the impedance between each of the secondary electrodes and the primary electrode, compare the impedances and based on the comparison, select the secondary electrode for coupling to the electrical device. In another detailed aspect, the controller is adapted to measure the conduction time of a pulse between each of the secondary electrodes and the primary electrode, compare the conduction times and based on the comparison, select the secondary electrode for coupling to the electrical device.
In a third aspect, the invention relates to a method for selectively ablating any one of a plurality of tissue regions within a biological site of a biological body. The method includes positioning at least one primary electrode adjacent one of the plurality of tissue regions. The method also includes positioning a plurality of secondary electrodes about the biological site. The method further includes selecting which of the plurality of secondary electrodes is positioned such that the tissue region adjacent the at least one primary electrode is interposed between the primary electrode and the secondary electrode. Additionally, the method includes providing energy to the primary electrode and maintaining the selected secondary electrode at a voltage level different from the primary electrode such that current flows from the primary electrode through the tissue region to the selected secondary electrode.
In a detailed aspect, positioning the plurality of secondary electrodes about the biological site includes positioning at least one secondary electrode adjacent the biological body at each of the anterior, posterior, lateral, and septal positions on the biological body. In another aspect, positioning the plurality of secondary electrodes about the biological site includes positioning the secondary electrodes adjacent the biological body in several locations about the outer surface of the biological body. In a further aspect, positioning the plurality of secondary electrodes about the biological site encompasses positioning at least one of the secondary electrodes within the biological body and about the biological site such that the tissue region is interposed between the primary electrode and the secondary electrode. In an additional aspect, positioning the plurality of secondary electrodes about the biological site includes positioning a plurality of secondary electrodes within the biological body and about the biological site. In a detailed facet of the invention, selecting the secondary electrode includes measuring the impedance between each of the secondary electrodes and the primary electrode, comparing the impedance between each of the secondary electrodes and the primary electrode, and selecting the secondary electrode with the lowest impedance. In a more detailed facet, measuring the impedance between each of the secondary electrodes and the primary electrode includes applying an electric pulse at the primary electrode, sensing the electric pulse at a first of the secondary electrodes, calculating the difference between the voltage of the pulse signal and the voltage of the sensed signal to determine the impedance, and repeating these steps for each of the remaining secondary electrodes.
These and other aspects and advantages of the invention will become apparent from the following detailed description and the accompanying drawings which illustrate by way of example the features of the invention.